the default encoding of a HTTP POST is ISO-8859-1.
else you have to look at the Content-Type header that will then look like
Content-Type: application/x-www-form-urlencoded ; charset=UTF-8
You can maybe declare your form with
<form enctype="application/x-www-form-urlencoded;charset=UTF-8">
or
<form accept-charset="UTF-8">
to force the encoding.
Some references :